1. Field of the Invention
The present invention relates to a method or maintaining the flowability of an acrylic polymer dispersion by which the flowability of a polymer dispersion comprising an acrylic water-soluble polymer used for applications, such as highmolecular flocculants, sizing agents, or paperstrength agents, can be maintained for a long period of time.
2. Description of the Prior Art
A water-soluble acrylic polymer has been commercially available in the form of an aqueous solution or a dry powder. However, both of them were poor in the workability and the handleability because the acrylic polymer is viscous in the form of an aqueous solution and is not fluid in the form of powder. In order to solve these problems, a proposal has been made on a method wherein fine particles of a water-soluble acrylic polymer are dispersed in an oil or an aqueous salt solution to prepare a low-viscosity liquid. For example, Japanese Patent Publication No. 39417/1977 discloses a water-in-oil type emulsion comprising 75 to 85% by weight, based on the emulsion, of an aqueous phase composed of 20 to 50% by weight, based on the emulsion, of a water-soluble addition polymer and 35 to 55% by weight, based on the emulsion, of water, 5 to 24.9% by weight, based on the emulsion, of a hydrophobic liquid, and 0.1 to 15% by weight, based on the emulsion, of a water-in-oil type emulsifier. Japanese Patent Publication No. 14907/1971 discloses polymerization of acrylic acid alone or together with a copolymerizable monomer in a solution having a pH value of 1 to 4 in the presence of an inorganic salt in an amount sufficient for precipitating a polymer with agitation at a speed sufficient for suspending the precipitated polymer. Further, Japanese Patent Laid-Open No. 123610/1986 of which the applicant is the same as the one pertaining to the present invention discloses polymerization of a cationic monomer represented by the following formula (1) alone or together with a copolymerizable monomer in an aqueous salt solution in the presence of a polymer soluble in the aqueous salt solution: ##STR1## wherein R.sub.1 is H or CH.sub.3, R.sub.2 and R.sub.3 are each an alkyl group having 1 to 3 carbon atoms, A is an oxygen atom or NH, B is an alkyl group having 2 to 4 carbon atoms or a hydroxypropylene group, and X.sup.- is a counter anion.
Japanese Patent Laid-Open No. 15251/1987 of which the applicant is the same as the one pertaining to the present invention discloses dispersion of a polymer of a cationic monomer represented by the above-described formula (1) alone or together with a copolymerizable monomer in an aqueous salt solution which does not dissolve said polymer, wherein another polymer soluble in the aqueous salt solution is added as a dispersant. Further, Japanese Patent Laid-Open No. 20502/2987 discloses polymerization of an acrylic water-soluble monomer in an aqueous salt solution with agitation while precipitating the formed polymer in the presence of a polyhydric alcohol.
However, in order to prepare a water-in-oil type emulsion as disclosed in the above-described Japanese Patent Publication No. 39417/1977, it is necessary to use a high-purity organic solvent as a dispersion medium. Use of such a solvent brings about an increase in the cost. The dispersion of a polymer in an aqueous salt solution disclosed in the above-described Publication No. 4907/1971 brings about agglomeration of particles and unfavorably loses its flowability in a week (usually in several hours) when allowed to stand even under favorable conditions. The methods disclosed in the above-described Japanese Patent Laid-Open Nos. 123610/1986. 15251/1987, and 20502/1987 are mainly applied to cationic polymers but cannot be applied to nonionic and anionic acrylic polymers without difficulty.